The mechanical strength of optical fibers in excess of 2.times.10.sup.5 psi is a desirable feature for certain specialized application (e.g., optical waveguides employing lengths of fiber).
Lack of mechanical strength is due to submicron flaws in the surface attributed, mainly, to chemical attack by atmospheric contaminants (e.g., moisture) during and after fiber drawing. Attempts to solve these problems have been studied by applying organic coatings to the fiber following the drawing of the fiber. Failure resulted because those organic coatings are not impervious to moisture or hydroxyl penetration. The penetration by moisture or hydroxyl resulted in reduced strength of the coated fiber during periods of storage and/or use.
Silicon nitride (Si.sub.3 N.sub.4) is an appropriate coating since it is resistant to moisture penetration and bonds securely to the silicon substrate fiber. Previous attemps to clad silicon fibers with Si.sub.3 N.sub.4 uses thermal (pyrolysis) techniques.
Thermal (pyrolysis) methods of influencing chemical processes lead, mainly, to the excitation of all degrees of freedom of the molecule. Both external (translational) and internal (electronic, vibrational and rotational) degrees of freedom are usually in thermodynamic equilibrium. In addition to there being an unproductive waste of energy, reactions with equilibrium excited molecules characteristically proceed in the direction of breaking the weakest bond, have a considerable percent of back reaction, many side reactions, and produce polymers.
Advantageous would be a method which employs laser photochemical reactions (LPR) to produce silicon nitride to clad freshly-drawn silicon fibers in an atmospheric controlled chamber. A particular advantage is recognized over the prior art vapor deposition method since the LPR method can be effected at room temperature.
An object of this invention to provide a method of coating optical fibers with Si.sub.3 N.sub.4 produced by laser photochemical reactions.
Another object of this invention is to deposit Si.sub.3 N.sub.4 on freshly-drawn silicon optical fibers in an atmospheric controlled chamber in a continuous operation which employs SiF.sub.4 and NH.sub.3 as the reaction gases.
A further object of this invention is to deposit Si.sub.3 N.sub.4 on freshly-drawn silicon optical fibers in an atmospheric controlled chamber in a continuous operation which employs SiH.sub.4 and NF.sub.3 as the reaction gases.